Conventional evaporative emission control systems, generally called evap systems, use a canister of adsorptive activated charcoal to store fuel vapors that would otherwise be lost to atmosphere, both the vapors vented from the carburetor bowl after engine shut down and the vapors formed in the fuel tank as the car sits. When the engine starts, these adsorbed vapors are then desorbed, or purged as it is generally known, by blowing air through the canister, with the air-vapor mixture then being fed to the engine for burning. A number of problems are presented by the conventional, engine burning method of stored vapor disposal. The purge air is generally pulled by manifold vacuum, which is strongest at just the time when the engine has the least ability to accept the vapors, that is, during idle or low power. Even if that problem is dealt with by a proportioning purge valve or through a separate source of purge air, the feeding of the purged air-vapor mixture to the engine must somehow be controlled in order to avoid adversely affecting driveability. Much of the patent activity in the evaporative emission control area treats the engine burning vapor disposal method as a given, and then proposes some system, often a complex system, to deal with the driveability problem. This is a problem that threatens to worsen in the future if and when new government regulations require that the tank vapors displaced during fuel fill, which are now generally vented to atmosphere, also be recovered. The capacity both of the conventional canister to store the extra vapor load and of the engine to burn it will be challenged.